In the recent past and up to the present day, there has been a proliferation of wireless electronic systems for use in a variety of applications and functions on vehicles such as cars, trucks, boats and aircraft. Each of these systems operates in a frequency band that is generally different from the other systems and, as a consequence, each requires an antenna operable in the particular frequency band. Such vehicles as airplanes, boats, and cars are generally equipped for one-way or two-way radio communication in various frequency bands, and also require the appropriate antenna for each particular communication band used. It can be appreciated that equipping a vehicle with a separate antenna for each frequency band of the several electronic systems can present a serious problem, especially in terms of available suitable space for mounting an antenna and for the space occupied by the antenna.
As a consequence, considerable effort has been directed to reducing the number of antennas required and to reducing the space which the antennas occupy. These efforts, at least in part, have been directed to producing a single antenna capable of handling two or more systems, i.e., a single multifunction shared aperture antenna. Such a multifunction antenna desirably should have a wide, although not necessarily continuous, bandwidth and also have both radiation pattern and polarization diversity. These requirements are due to the fact that different electronic systems operate at different frequencies and generally require different radiation patterns and polarizations. In the case where the antenna is to be mounted on aircraft, or missiles, aerodynamic considerations require that the antenna protrude as little as possible from the vehicle surfaces, e.g., the fuselage. In the case of an automobile or truck, there is a need to reduce protrusion of the antenna for protection thereof against breakage or other damage or vandalism. Other factors such as privacy, security and aesthetic appearance are also to be considered. In all such cases there has been an effort to develop conformal antennas which may be mounted on or integrated into the fuselage of an airplane or the rooftop of an automobile, for example, and which conform to the contour or profile thereof.
Unfortunately, present day multifunction antennas which satisfy the aforementioned criteria are virtually non-existent, although a multifunction antenna is shown in U.S. Pat. No. 4,711,488 of E. J. Perrotti. Most antennas which satisfy the requirement of comformability do not have bandwidths that exceed thirty percent (30%) hence the range of frequencies accommodated by the antenna is severely limited. In addition, most multifunction antennas at the present time are usually of the high directivity type and incapable of adequate omni-directional or broad beamed operation. See, for example, U.S. Pat. No. 5,160,936 of Braun et al. and "6 to 18 GHZ Transmit/Receive Modules for Multifunction Phased Arrays," 1989 IEEE MTT-S International Microwave Symposium Digest, Vol. 1, pp. 115-118, Long Beach, Calif., Jun. 13-15, 1989.
In U.S. patent application Ser. No. 07/962,029, filed by the present inventors now U.S. Pat. No. 5,313,216, issued May 17, 1994, the disclosure of which is incorporated herein by reference, including the discussion of the prior art, there is shown a multi-octave microstrip antenna which is conformable. More particularly, that application shows a spiral mode microstrip (SMM) antenna having a broad bandwidth (presently approximately 900%), conformability, and a low profile. The antenna basically comprises a spiral-mode antenna element and a substrate for spacing the element a selected distance from a ground plane. Preferably, the antenna element comprises a thin foil of conducting material, such as copper, having a frequency independent pattern form such as a spiral, sinuous, tooth, or log-periodic pattern. In addition, an optional loading material is positioned adjacent the periphery of the patterned element, and the antenna feed is located at the center of the pattern. The antenna as described has a bandwidth characteristic comparable to that of prior frequency-independent antennas, and also has conformability to any mounting surface. Such an antenna also represents an improvement over prior art antennas in that it is low profile and conformable, and possesses both radiation pattern and polarization diversity.
Despite the improvements in performance characteristics resulting from the SMM antenna's unique configuration, there are certain frequency bands in use in automobiles, for example, where the performance of the SMM antenna can be improved, thereby achieving a greater multifunction characteristic.